Posted: Wed Jul 30, 2008 5:40 pm-
6 weeks should more than enough if you supply all the required documentation but the sooner the better.
I would wait for the visa before buying the tickets. There's no need for a flight itinerary to apply for the visa (I got mine and I still haven't booked a flight yet). I actually think they recommend not to buy the tickets until the visa is issued.
So if your application is complete and with all the necessary documents (offer letter from uni, proof of funds, etc) it should take less than 1 week.
Also check with your university if your course needs an ATAS (Academic Technology Approval Scheme) certificate (which is required for most science and engineering postgrad research degrees, for non EU/EEA citizens). You can usually find this info on the university website.